Owing to the increasing scarcity of non-renewable energy reserves such as coal, petroleum and uranium, increased use is being made of alternative nondepletable energy sources, such as photovoltaic energy. Single crystal photovoltaic devices, especially crystalline silicon photovoltaic devices, have been utilized for some time as sources of electrical power because they are inherently non-polluting, silent and consume no expendable natural resources in their operation. However, the utility of such devices has been limited by problems associated with the manufacture thereof. More particularly, single crystal materials are difficult to produce in sizes larger than several inches in diameter, are thicker and heavier than their thin film counterparts, and are expensive and time consuming to fabricate.
Considerable efforts have recently been expended to develop systems and processes for preparing thin film amorphous semiconductor materials which encompass relatively large areas and which can be deposited so as to form p-type and n-type semiconductor layers for the production therefrom of thin film n-i-p type photovoltaic cells which are substantially operatively equivalent or superior to their crystalline counterparts. It should be noted at this point that the term "amorphous", as used herein, is defined to include alloys or materials exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions.
Amorphous thin film semiconductor alloys have gained acceptance for the fabrication of electronic devices such as photovoltaic cells, photoresponsive devices, photoconductive devices, transistors, diodes, integrated circuits, memory arrays and the like. This is because the amorphous thin film materials (1) can now be manufactured by relatively low cost continuous processes, (2) possess a wide range of controllable electrical, optical and structural properties, and (3) can be deposited to cover relatively large areas. Among the amorphous semiconductor materials exhibiting the greatest present commercial significance are amorphous silicon and amorphous silicon-germanium alloys.
The assignee of the instant invention has previously developed a process and apparatus for the continuous glow discharge deposition of successive layers of doped and intrinsic amorphous silicon alloy material onto a web of substrate material, such as stainless steel, so as to fabricate, in an inexpensive process specifically adapted for mass production, a photovoltaic cell. In said process, isolated chambers are dedicated for the successive deposition of each discrete layer of amorphous silicon alloy material. Such continuous processing apparatus is disclosed in the following U.S. patents, the disclosures of which are incorporated herein by reference: U.S. Pat. No. 4,400,409, for "A Method of Making P-Doped Silicon Films and Devices Made Therefrom"; U.S. Pat. No. 4,410,598, for "Continuous Amorphous Silicon Solar Cell Production System"; and U.S. Pat. No. 4,438,723 for "Multiple Chamber Deposition and Isolation System and Method."
In machines previously built by the assignee of the instant invention, the photovoltaic cells included six discrete layers, i.e., a first n-type layer, a first intrinsic layer, a first p-type layer, a second n-type layer, a second intrinsic layer and a second p-type layer. In this manner, a "tandem" photovoltaic cell was fabricated comprising two distinct solar cells optically and electrically connected together in series. By varying the optical band gap of each of the layers of intrinsic silicon alloy material, different wavelengths of light present in the incident solar spectrum was absorbed in each of those intrinsic layers. And due to the fact that multiple intrinsic layers are available for the absorption of photons of incident radiation, each intrinsic layer was relatively thin so that the internal electrical field generated by the doped layers acting on the charge carriers photogenerated in the intrinsic layers was maximized and operated to efficiently separate and collect those charge carriers before recombination occurred.
In the glow discharge deposition of amorphous silicon alloy material, specific precursor gas mixtures are fed into each of the discrete deposition chambers. The r.f. electrical field established interiorly of the deposition chamber decomposes said gas mixture and causes the disassociated gas fragments to be deposited onto the surface of a grounded substrate. For instance, a mixture of silane, hydrogen, argon and silicon tetrafluoride is fed into the intrinsic deposition chamber and a mixture of silane, hydrogen and boron tetrafluoride is fed into the p-type deposition chamber operatively disposed adjacent thereto and in operative communication therewith. Since the process is a continuous one in which deposition occurs in discrete chambers, an opening between those adjacent deposition chambers must constantly be maintained. However, the boron from the p-type deposition chamber represents an impurity if introduced into the intrinsic deposition chamber and accordingly cannot be permitted to enter said intrinsic chamber through said opening without deleteriously affecting the overall performance of the photovoltaic cell.
In order to prevent the diffusion of boron into the intrinsic deposition chamber, the assignee of the instant invention has also previously developed "gas gates" which are adapted to be operatively positioned between said adjacent chambers. These gas gates operate by establishing a flow of an inert gas (or a gas which is common to the layers of amorphous silicon alloy material deposited in both of the contiguous, operatively interconnected chambers) in a direction opposite to the direction of flow of the "contaminant" gas. By designing the length and height of the gas gate to be sufficient, molecules of the diffusing contaminant gas will encounter and collide with molecules of the inert gas, thereby ensuring that the unused contaminant gas will be removed through the exhaust conduit in the deposition chamber in which it is introduced rather than diffusing into the adjacent deposition chamber and being deposited as part of the precursor deposition gas mixture introduced into said adjacent chamber. It is critical to note at this juncture that these gas gates could only prevent contamination of one deposition chamber from free-flowing gaseous species present in the adjacent chamber. These gas gates offered no protection from contaminants chemically absorbed on the surface of the substrate passing between the adjacent chambers. It is one of the objects of the instant invention to prevent contamination due to chemically absorbed species.
Those of ordinary skill in the art of solar cell fabrication, reading the instant specification, will readily appreciate that the lengths of each of the deposition chambers are proportional to the thickness of the discrete layers of amorphous silicon alloy material deposited therewithin. For instance, the doped layers of amorphous silicon alloy material are each about 100 angstroms thick, whereas the thickness of each of the intrinsic layers of amorphous silicon alloy material, in a two cell tandem configuration, will be about 1000 to 4500 angstroms thick. Correspondingly, the length of the doped deposition chambers will be about three feet in length. It should also be appreciated that the gas gates will also vary in length from about eight inches for the gas gates separating and isolating the gaseous contents of the doped and intrinsic deposition chambers; and sixteen inches in length for the gas gate separating the contiguous doped deposition chambers in which the top layer of the first stacked cell and the bottom layer of the second stacked cell are deposited.
In addition to the chambers already described hereinabove, the deposition apparatus also requires a payout chamber from which the web of the substrate material is dispensed and a take-up chamber in which the web of substrate material with the layers of amorphous silicon alloy material deposited thereupon is wound for downstream processing into completed photovoltaic modules.
Finally, it is also important to mention that an additional plasma process occurs to help improve the performance of the photovoltaic cells. Again, note that each stacked cell is of a p-i-n-type configuration. In such a configuration, the interface which exists between the p doped layer of amorphous silicon alloy material and the intrinsic layer of amorphous silicon alloy material forms the "major semiconductor junction" of the photovoltaic cell. In order to affect the best possible semiconductor junction condition, the assignee of the instant invention routinely subjects said intrinsic layer to a plasma treatment prior to the deposition upon that surface of the p-doped layer. This plasma treatment passivates dangling, missing, or broken bonds or otherwise decreases the density of defect states present in the host matrix of the amorphous silicon alloy material. In order to effectuate that plasma, the deposition apparatus has heretofore incorporated an additional elongated chamber operatively disposed between the p-doped chamber and the intrinsic chamber. Of course, such a plasma chamber serves to add yet further length to the deposition apparatus, especially considering the fact that it is preferred to passivate the deposition surface existing between each of the interfacial layers of the tandem photovoltaic device.
As mentioned hereinabove, stacked, large area photovoltaic devices are currently being manufactured by the assignee of the instant invention on a commercial basis by utilizing the previously referenced, continuous deposition, roll-to-roll processor. That processor is characterized as having a 1.5 megawatt capacity in its annual output of photovoltaic power. Said 1.5 megawatt processor is adapted to produce tandem photovoltaic cells which comprise two stacked n-i-p type photovoltaic devices disposed optically and electrically in series upon a stainless steel substrate. The processor currently includes six operatively interconnected deposition chambers, each deposition chamber adapted to sequentially deposit one of the layers of silicon alloy material from which the tandem device is fabricated. With the addition of all of the aforementioned chambers, the 1.5 megawatt processor has a total length of approximately 40 feet. The assignee of the instant invention is currently designing a 10 megawatt machine for the continuous fabrication of significantly higher outputs of photovoltaic power. In order to produce an annual output of 10 megawatts of electrical power, the length of the machine will be increased significantly.
A first reason for the increased length of the processor is that it will be specially configured to fabricate three stacked photovoltaic cells; therefore, the processor will require 9 dedicated deposition chambers instead of the six dedicated chambers required by the 1.5 megawatt processor. A second major factor in determining the length of the processor is the aforementioned fact that the length of each chamber is dependent on the thickness of the layer being deposited therein. The thickness of the material is dependent upon the rate of deposition of the particular gas mixtures and the speed of the web of substrate material passing through that chamber of the processor. Assuming that the rate of deposition remains substantially constant, the web speed will have to be kept constant and the 10 megawatt processor will be about seven times longer than the 1.5 megawatt processor. Even assuming that the presently employed one foot wide web of substrate material were increased in width to two feet, a scaled-up version of the processor would be over 100 feet in length.
It should therefore be abundantly clear to the reader that, as the 1.5 megawatt processor is scaled up to higher throughput capacities, it becomes an economic necessity to substantially reduce the overall length thereof. It is to the end of decreasing the overall length of the roll-to-roll deposition apparatus that the second major advantage of the instant invention is directed.
The third and final major advantage of the instant invention resides in the plasma cleansing and/or passivation of at least the major junction interface formed between two of the successively deposited layers of amorphous silicon alloy material of each of the stacked cells of a photovoltaic device.